In a medium sized aircraft, such as a 100 passenger aircraft, two aircraft engines and an auxiliary power unit (APU) are often provided. Conventionally, the main engines are started by air turbine starters, requiring valves, ducts and other equipment in the tightly packed engine nacelle. Further, the APU is typically started by a DC starter motor which only provides a useful function for a very short period of time over each APU engine cycle. Also, the starter motor is a high maintenance item owing to the use of brushes and an overrunning clutch therein.
Of late, there has been a desire to provide variable frequency power generation capability on the main engines. When the power rating of the generator is sufficient, main engine electric start becomes an option. This capability allows significant amounts of equipment to be removed from the aircraft, particularly in tightly packed areas, such as the engine nacelle.